I like making a puzzle toy out of the milk jugs too. I cut just enough of a slit in the bottom that I can squeeze the jug (opening the slit) to pop a couple tennis balls in, then let it pop back to normal. Kaiju loves wrestling the jug to the floor and figuring out exactly how to press and angle it to knock the tennis balls out.

It becomes a tougher one when you get thicker containers (like the one for Simply Orange juice) that are a bit harder to work the balls out of.

Cardboard boxes are good for "find the toy" too. On rainy days, I'll fill a room with a bunch of my extras, some overturned or on their side and some right side up, hide his ball while he's waiting in the next room then release him and tell him to find it.